System of transmission over loaded lines



Dec. 14 1926.

. H. NYQUIST SYSTEM OF TRANSMISSION OVER L|0ADED LINES 3 Sheets-Sheet 1INVEN TOR A TTORNE Y Dec. 14 1926.

H. NYQUIST SYSTEM OF TRANSNI'ISSION OVER LOADED LINES Filed March 281925 ATTORNEY Dec. 14 1926.

H. NYQUIST SYSTEM OF TRANSMISSION OVER LOADED LINES Filed March 28 19235 Sheets-Sheet 5 515 2.900 F; 10" F1 to n INVENTOR ZMgmsZ A TTORNEY mam.nYeUIsT, :o'r nniv znunsm, New Yon-K;- se-acme to AMERICAN anemone ,Ann TELEGRAPH comma- Y, A coaeo ta'i ion on Y-ORK'.

svsrnivr be rritn 'iki'series101v :ovnn toannn m nes.

' Application mai man 25;, 1323} "seriarnb. (2 3,289. 1 a

Ano'bj'ectof my invention is provide without harmful distortion. Theseand vari- I one oth'erjobjects of my invention will loe comeapparent onconsideration of a limited- It will be understood that the following forutilizing the ayailahl'e frequency range on a loaded line for,transmission Without harmful distortion. ;Another objectis to. improvethe quality of voice transmission on" a loaded line by separating 'ofi"thehigher frequency components below the critical-frequency ofthel'oaded line, and to utilize these higher frequency componentsfor"telegraph I of tWo k1nds.first, the components of various freq'uen'ciesmay be attenuatedover the 1 .7 05. components may be retarded.d1fierently;.f- The first of these efiects, vizunequalfiatten channelsin a manner. to effect transmission number of examples ot'spfecificpractice accordingto the inventionwhichI have chosen to describe in thefollowing specification; a

description relates'inore particularly to these examples of my inventionand that the scope of the invention will be pointedout in the" appendedclaims. I V I, H

Referring to the drawings, Fig.- ljis 'a T curve "diagram that Willloe.referred to in explaining the; character of the distortion on a loadedline; Fig. 2 is adi'agram of simple apparatus to. illustrate theprinciple involved, my'invention. Figs. 8 and .4 show the character ofosoillograms obtained in connection with the apparatusof Fig. 2,. I Fig.5 jshoWs apparatus for. the practicezof my invention,in connection jwitha simple; four-Wire transmission line; Fig. 6 is' a f diagram ofterminal apparatus for thepraca sponding to the .-.inFig.6."

ticev oii my invention in connectionwith phantom and'side circuitsoperated on the V fourewire principle; and Fig, T is a diagram ofapparatus at a repeater station. correa terminal apparatus shown Intransmission over a loadedline there is certain critical frequencyfixed,rather definitely by the design of the line, such that a the linetransmits comparatively freely. cur,

rentsof any frequency below the critical frequency,ibut the linetransmits'higher frequenciesfonlylivithextreme attenuation or Ipractically it does. not transmit them at all.

' As an example, a loaded" line compri sed in a cable may have"itseritical frequencyfat f 560(1 'cycles .per second. This means fthatfrequencies belotvf5 600 will loejtransmitte'ch ,oomparatively freelyover the line While frequencies above." 5600 appliedto one end;

-.higher-- an ponen'ts.

.ofcthe linetw ill prod ea aaeaia en-J."

Thef i requency range essentials-for good voice transmission extends upto about 250G:

cycles per "second; 'but oif'course the voice frequency components canloe transmitted Without distortion.

For voice transmission, distortion may loe line; in different degrees,,and second, the

nation, can overcomehythe'use of an attenuation equalizer, and thisexpedient' is Well Worked outin theory and practice. The

second difficulty, unequal retardation of-the components, may besomewhat; serious in the, case. offal loaded line. Ilfvariousifrequ'encies aresent over a loaded-line, and if the time of 'arriv'alfor'the low frequencies is taken as' zerol, then the timeitorother fr lquencies, when plotted as ordinate against the frequency as" abs'cissa,.Will-give a curve such as that'shown inFig'I, fOn the upper. scale ofabscissaefthe ratio'olfthe variahle frequency .tothe criticalfrequency-is ;laid;

so l

03. The time of transmissionifor frequen- .c'ies approaching 0. is takenas unity for the; scale of ordinates; The

be" a j 'ne't improvement I by cutting 0H these qua io of the;

l he objectionableeffect tofithegretarded q n y compone s may-t exhi dn7. another way lflncarrier "current' telegraphy it will be seen thatoverla'trequ enfcy range h mo to -between and i017 ;ofv the entirerange. upfto the-critical frequency, thelretardatio ii,only attainsa-bout 1O%, but for frequencies close to the criticalfrequency the,retardation increasesrapidly andinay bye-:1: come as greatfasf50 ormore.Hence, for" gvoiceftransmission itmay' be that therewill d.muchfretarded:frequency 3 1 the dots and dashes consist oftrains of alter-I.jnating' current of i a certain; carrier frequency' 1 pp e ns Z t esend n key i j les i. 'But when a steady alternating I EgjiM Fgjisapplied to alinc, the resulting current comprises various frequencies atthe outset aside from the fundamental frequency, and these various otherfrequencies take an appreciable time :tojdie away, so that thereis acon:

siderableiinterval of time after the steady alternating E. F. is applieduntil the steadystate alternating current is reached.

Then, supposing that the key has been closed 7 received signahandthedifferential retardato make a dash or a dot, when the key is openedto terminate :the signal. impulse, the I current will die down 1n anirregular manher taking some time.

.Three possible causes-may. be assigned for the length of the period ofbuilding up the current at the receiving end onfthe application of asteadyulfl; M. F. at the sending end, In the first place the loaded,line cir cuit has a definite cut-off frequency, the.

critical frequency, above'whiclrno current whatever is transmitted.Second, the transmitted components are unequally retarded in l ponents.

transmission depending on their frequency.

' Third, the attenuation for thetransmitted frequencies difiers from onevalue of frequencyto another. 1 H

The first ofj'these en ects'is known to be relatively unimportant forthe reason that, if important, it would be present in a rela tivelyshort transmission line, whereasobservation shows that the transienteffects in a short @loaded line are relatively insignificant. The thirdeffect can be overcome by the use of attenuation equalizers, but afterthis is done there will be very considerable transient effects in longloaded lines, and

r they must be ascribed principally to the second of the three causesthat have just been enumerated. This means that when a sinusoidal'E; MrF is suddenly impressed on a 'cir cuit, theresulting current may beregarded as'inade up of a large number of components varying infrequency from Oto infinity. These components of 5 different 2 frequency{-travelalong the transmission line at different rates of speedandrecombine at the distant end to form the received current'which isaccordingly considerably distorted as compared with the transmittedcurre'nt'on account of the unequal retardation.ofthecoma This unequalretardat onis'shown -inFig.1. I 7

Now suppose for an illustrative example thatl the carrier currentfrequency, that is the frequency of the sending M. F. issuch thatf/f=0.75,'and that a band filter is employed' whichcuts off all thefrequencies be- 1 low 0.7 ;and all above 0. 8. -From the curveinFig.1;it will be apparent that the great;

est relative retardation within this frequency range is-0.051*Withftliis small relative de day we shouldf expect only a small distortion eifectiin the receivedsignalj From the foregoing it will beapparent that telegraph transmission-shouldv be improved by the use ofaband filter of narrow range including the carrier frequencyat itsmid-range. it should be remembered, how ever, that as the frequencyrange for the tion effect of the line will befsubstantially negligible.v A specific illustration of the principles that have been explained maybe given by means of theapparatus shown in Fig. 2. At. G isanalternatingcurrent generator with frequency 2,000. Upon the closure ofthe key K, an E. M. F. ofthis frequency will be applied to the lineL.In. this example the line L consists of three loops of loaded line eachextendingSl' miles and back, with repeaters R as shown, and the outputfrom the last repeater R goes to one element of the osciliograph Thusthe length of the repeatere'dline is six times 87 iniles or 522 miles.Anotherelement of theoscil lograph O is operatively connected-to thesending circuit throughxthe sending key K. 7 By means of the switches Sthe band filter BF can be interposed'when desired. Its freouency rangeis from 1900 to 2100.

When the key K is closed with the switches S in the position shown in 2,all the components in therangeshown in 1 are putjupon the line and mayget through to the receiving end n various degree. In this case theoscillogram has the character shown in Fig. The film moves from right toleft, and the oscillograph mirr *hichf' is associated witlrth-e sendingkey-"K gives theline 11 until the key .Kisclosed, wherei iiupon at 12sinusoidal curve appears, whose On the envelope isshownin dotted lines.opening of the key, 1"; thesinusoidal curve ends and the steadyopen-circuit cond tion is, indicated by the line 11 The period closureof thekey is seen "to he-0.11 7 isec. The escillogran for; the received.curi t shown at the lower part of Fig. t'lhe interval betweenthe-closure ofthe sending key K and the first' re'ceived impulse0.05'4'5 sec. Tihen comesa building upperiod, 'of 0.02% sec. Theenvelepeiofthe oscillogram is indicated,indotted lines, and it will beseen that at the end of thereceived sigr nalinipulse there is a t 'ing0.0262 sec. Ne t switches S are thrown in F 2. so as interpose; the bandfilterwhich; will out. tall-the component frequenciesbelowand'ahove2i00, v,Clhe effect is shownin F g. 4 where with a se d7, ingimpulse'of 0.113 see, the building i p periodis reduced to 0.0065 andthe'dying down: transient disturbance :p'eriod g-is; onl 0.0070sec. Thusfthe period of .thef initial nsient eifect last- "ent'is I transientdisturbance is reduced to about interposlng the handy filter, and

a fourth ,b I

the perioc of the terminal transient disturbance is also only about onefourth as' long as before. The interval after the hey.

, is closed until the first received impulse is 'pro'vemnt shown in Fig.e'higher felt is longer with the filter interposed, as

would naturally be expected, 0.0724 sec, as;

With .the' im-' telegraph speed can be attained, because the initial andterminal ends-of thereceived signal imcompared with 0.0576 sec.

pulse will be'less distorted. V v r t The oscillograms of Figs. 3 and 4were actuallytaken with a medium heavy loaded line that cut off at about2800 cycles per sec- 7 cycles. per second, making fover the line L ondand the frequencv employed was 2000,

InyFigfdI have shown how my 1nven- .tion maybe practicedin connectionwith a loaded line operated according tothe fourwire system by which twoconductors of one circuit are employed for sending, and thetwoconductors of another circuit are em ployed" for receiving. Transmitted,voice currents originated by the subscriber, at Sb: go, as shown by thearrows, throughthe repeater R and the low-pass filterLF to the line L.The. critical frequency of the lowpass filter LF is 2500.

Return voice currents comlng from the distant subscriber go through thelow-pass filter and the repeater, as shown bythe arover'the rangeindicated cycles per second. This rows, to the subscriber Sb.

Various setsof telegraphtransmitting apparatus each designated throughthe respectiveband filters BF in multiple to thecircuit L {whichconnects r I through the repeater ,40

R and the high-pass filter HF withjthe line L. The. critical fre quencyfor the higlrp'ass filter lllF is 2500. Each of the band filtersBFtransmits freely legends on the drawings. F orexample one of themtransmits between 2650'and 2850 carrier. frequency of- 2750. i V V.Received carrier current telegraph signal impulses coming inover thelinelL are 'blocked by the low-pass "lltel LF but passed by thehigh-pass filter HF, as shown by the arrows, and then they go throughthe repeater to the branch circuit L V cuit L The Va-ri ous telegraphreceiving sets Re are connected through respective band filters withthis cir- These band filters pass the ffrequency ranges indicated intherespectiveatthree shown 7 range is 2 900 to 3100.

tached legends. It will be seen that if a telegraph operator at'thedistant station. is sending on acarrier frequency of 8,000

. cycles per second, his impulses will be block-ed by all'the bandfilters connected to f the line L' except the middlefilter of the inFig; 5 whose-transmitting be practicable to channels,

T7 are connected erable distortion. in the respective corresponds to y a7' 'tion of my inventionreceiving. The usual telegraph apparatus isphantom units as indicated by} the apparatus designated M; Connectedwith each phanmitted voice currents V peaters andgfilter sets F to thephantom unit as lndicated by 'The telephonej subscriber at S5 in r andthe party with whom he is talking at the other end of the line will havean im-' provement of the voice quality; because only thosefrequenciesbelow' 2500 will be 1nv Volved in their conversation. vThehigher frequencies would be unequally retarded and would thereforeproduce distortion, but

s by the use of the-highand low-pass filters,

they will be kept viatus involved incarrying on the telephone 1conversation. V- a 1 Suppose the critical frequency :for; the loadedlines involved is 5600; A

out of the terminal appas frequency scalefrom 0 to 5600 isla-idoff atthe lower part of Fig, 1. Becauseof thesteepness of the curve as itapproaches 5600,1'it winner:-

but the range from 4600 down to 2500 will be available.

telegraph channels may respective car a000, 3250, a500, 3750, 4000, 4250 and 4500. Each of theseeight channels, will be esbe provided ywithtablished by the use of a respective band filter with transmittingtending frequency vto 100 cycles above. ample, one band filter-v willfrequency range we Thus, for exrier .frequency 8500.

to 3850 correspond- Telegraphtransmission in each of these use this partfortelegraphl In this range eight rier current frequencies at 2750,.

v have its range. from 3400110 3600 corresponding to the car- The nextfilter will have its rangefrom 3650 ring 'to a carrier current frequencyof 37 50 'and'soon;

from cycles below the carrier channels will be better thanz if the Wholefrequency range .of theqloaded 'line [below 5600 were open as a singlechannel, because if that were the case;

I the components of frequency remote from the carrier'would be wouldgive consrd pluralityof conductors in, acable wherethe I pairs ofconductorsareoperated as sidecirphantom circu ts. A

cuits and in quads as terminal station is shown in Fig, 6. Compositephantom units one above for sending tom unit P are'thre'e local circuitsas shown.

Three subscribers sets are 7 indicated eamy the symbol Sb. Each isconnected with raj apparatus HY; Transgo "through the Teourwi'reterminal the arrows. It WiIl are-shown at P, the 1 andthe onebelow for superposed metallic; taken off from the at once be seen alsohow received currents coming in over the lower right-hand quad -go tothe subscribers sets Sb.

The telegraph transmitting sets are each indicated by the symbol TS,each such symbol inclicating eight transmitting sets with theirrespective band filters. These are all connected by theline L with therepeater RZBL being the same branch circuit asdesignatcd "by thatcharacter in Fig. 5.] From the reconductor pairs are shown, one for eachsit e Iclaim: 1. In combination, a loaded line, filters to in saidhigher range below the'critical frecircuit and one for the phantomcircuit. Each pair of conductors leads from the phan tom unit to afilter set which separates the low and high frequencies. These areainplified in the respective repeaters provided for that purpose, andcombined in the filter set F- and returned totl e' phantom unit By theuse of my invention the quality of 1 transmission for both telephone andtelegraph messages can be improved in connection with loaded lines. Thisimprovement is gained without sacrificing the available frequency rangefor carrier current transmission, but on the other hand-this range isfully utilized.

separate component currents of various frequencies thereon into apracticable voice range and a higherrangeand band filters to establish aplurality of telegraph channels quency of the loaded line.

.2. In combination, a line having the property of retarding currents ofdifferent fre 'quencies in substantially unequal degree, and means toestablish a plurality of channels, each of comparatively. narrow fre-Lquency range whereby the differential re} tardation within each suchsubstantially negligible.

3. The method of improving carrier current telegraph transmission on aloadedline, which consists in putting currents of various frequencycomponents on the line and separating by at least one filterthose'components whose frequency is considerably different from thecarrier frequency.

' 4." The 'method of utilizing a range will be frequency range withsubstantially unequal retardation 5 at various frequencies whichconsists in 'sepa'rating'it into channels ofdifferent width 5 I with thenarrower channels in the'part of the range where the differentialretardation v is greater. V v

5. In combination, a loaded line having a certain critical frequencybelow-which it transmits current components with different retardationaccording to their frequency'and above which it transmits substantiallyno current, filters to separate current components of differentfrequencies on the, line into a range below approximately 2,000 .cyclesper second and a range above that frequency and further to separate thelast mentioned range into a plurality of tele- V graph channels.

In coinbinatlon, a loaded line having the property of cutting off allcurrents of frequencies above a certain critical' fre-" 'quency and ofsubstantially differently retarding the components below that frequency,and means to establish a plurality of channels each of comparativelynarrow frequency range whereby the diiferentialretardation within eachsuch range will-be sub- I stantially negligible.

7. The method of improving the quality of transmission by carriercurrent on a loaded line whlch consists in-employing acOmparatiVely'narrOW range of frequencies for the currentcomponentswithin. a single" channel and separating therefrom currents ofother frequencies; I

v In testimony whereof, I have signed my name to thisspecification this27th day of March, 1923. p

'HAnRY YQUIsT,

